Composition comprising low density microspheres

ABSTRACT

Low density microspheres, methods for preparing same, and use of same as contrast agents are described. The microspheres have a void having a volume that comprises at least about 75% of the total volume of the microspheres, and which contains a gas or the vapor of a volatile liquid selected from the group consisting of aliphatic hydrocarbons, chlorofluorocarbons, tetraalkyl silanes and perfluorocarbons.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.10/280,844, filed Oct. 25, 2002, now U.S. Pat. No. 6,773,696, which is acontinuation of U.S. Ser. No. 08/878,233, filed Jun. 18, 1997, now U.S.Pat. No. 6,528,039, which in turn is a continuation of U.S. applicationSer. No. 08/594,269, filed Jan. 30, 1996, now abandoned, which is adivisional of U.S. application Ser. No. 08/456,738, filed Jun. 1, 1995,now U.S. Pat. No. 5,527,521, which is a divisional of U.S. applicationSer. No. 08/449,090, filed May 24, 1995, now U.S. Pat. No. 5,547,656,which is a divisional of U.S. application Ser. No. 08/116,982, filedSep. 7, 1993, now U.S. Pat. No. 5,456,900, which is a divisional of U.S.application Ser. No. 07/980,594, filed Jan. 19, 1993, now U.S. Pat. No.5,281,408, which in turn is a divisional of U.S. application Ser. No.07/680,984, filed Apr. 5, 1991, now U.S. Pat. No. 5,205,290.

BACKGROUND OF THE INVENTION

Computed tomography (CT) is a widespread diagnostic imaging method whichmeasures, in its imaging process, the radiodensity (electron density) ofmatter. This radiodensity is depicted using CT in terms of HounsefieldUnits (HU). Hounsefield Units, named after the inventor of the first CTscanner, reflect the relative absorption of CT X-rays by matter, theabsorption being directly proportional to the electron density of thatmatter. Water, for example, has a value of 0 HU, air a value of −1000HU, and dense cortical bone a value of +1000 HU. Because of thesimilarity in density of various tissues in the body, however, contrastagents have been sought to change the relative density of differenttissues, and improve the overall diagnostic efficacy of this imagingmethod.

In the search for contrast agents for CT, researchers have generallysought to develop agents that will increase electron density in certainareas of a region of the body (positive contrast agents). Barium andiodine compounds, for example, have been developed for this purpose. Forthe gastrointestinal tract, barium sulfate is used extensively toincrease the radiodensity of the bowel lumen on CT scans. Iodinatedwater soluble contrast media are also used to increase density withinthe gastro-intestinal tract, but are not used as commonly as the bariumcompounds, primarily because the iodine preparations are more expensivethan barium and prove less effective in increasing radiodensity withinthis region of the body.

Despite their widespread use, however, barium and iodine compounds aresuboptimally effective as gastro-intestinal contrast agents for CT. Forexample, if the concentration is too low, there is little contrast.Conversely, if the concentration is too high, then these radiodensecontrast agents cause beam hardening artifacts which are seen as streakson the CT images. It is also difficult to visualize the bowel mucosawith either the barium or iodine contrast agents.

In an attempt to improve upon the efficacy of contrast agents for thegastrointestinal tract, lipid emulsions that are capable of decreasingelectron density (negative contrast agents) have been developed. Becauselipids have a lower electron density than water, lipids provide anegative density on CT (a negative HU value). While these lipidemulsions appear to be more effective than the barium and iodine agentsat improving visualization of the mucosa of the bowel, these contrastagents have limitations. First, there is a limitation to theconcentration of lipid which a patient can tolerably drink, which puts alimit on the change in density (or HU) which the lipid based CT contrastagent can provide. Lipid emulsions are also frequently expensive.Furthermore, these lipid formulations are generally perishable, whichprovides for packaging and storage problems.

New and/or better contrast agents for computed tomography imaging areneeded. The present invention is directed toward this important end.

SUMMARY OF THE INVENTION

The present invention is directed to computed tomography imaging, andmore particularly to the use of a contrast medium comprising asubstantially homogeneous aqueous suspension of low density microspheresto image the gastrointestinal region and other body cavities of apatient. In one embodiment, the low density microspheres are gas-filled.

Specifically, the present invention pertains to methods of providing animage of the gastrointestinal region or other body cavities of a patientcomprising (i) administering to the patient the aforementioned contrastmedium, and (ii) scanning the patient using computed tomography imagingto obtain visible images of the gastrointestinal region or other bodycavities.

The present invention is further directed to methods for diagnosing thepresence of diseased tissue in the gastrointestinal region or other bodycavities of a patient comprising (i) administering to the patient theaforementioned contrast medium, and (ii) scanning the patient usingcomputed tomography imaging to obtain visible images of any diseasedtissue in the patient.

The present invention also provides diagnostic kits for computedtomography imaging of the gastro-intestinal region or other bodycavities which include the subject contrast medium.

DETAILED DESCRIPTION OF THE INVENTION

A wide variety of different low density micro-spheres may be utilized inthe present invention. Preferably, the microspheres (which are smallspheres having a central void or cavity), are composed of biocompatiblesynthetic polymers or copolymers prepared from monomers such as acrylicacid, methacrylic acid, ethyleneimine, crotonic acid, acrylamide, ethylacrylate, methyl methacrylate, 2-hydroxyethyl methacrylate (HEMA),lactic acid, glycolic acid, ε-caprolactone, acrolein, cyanoacrylate,bisphenol A, epichlorhydrin, hydroxyalkylacrylates, siloxane,dimethylsiloxane, ethylene oxide, ethylene glycol,hydroxyalkyl-methacrylates, N-substituted acrylamides, N-substitutedmethacrylamides, N-vinyl-2-pyrrolidone, 2,4-pentadiene-1-ol, vinylacetate, acrylonitrile, styrene, p-amino-styrene,p-amino-benzyl-styrene, sodium styrene sulfonate, sodium2-sulfoxyethylmethacrylate, vinyl pyridine, aminoethyl methacrylates,2-methacryloyloxy-trimethylammonium chloride, and polyvinylidene, aswell polyfunctional crosslinking monomers such asN,N′-methylenebisacrylamide, ethylene glycol dimethacrylates,2,2′-(p-phenylenedioxy)-diethyl dimethacrylate, divinylbenzene,triallylamine and methylenebis-(4-phenyl-isocyanate), includingcombinations thereof. Preferable polymers include polyacrylic acid,polyethyleneimine, polymethacrylic acid, polymethylmethacrylate,polysiloxane, polydimethylsiloxane, polylactic acid,poly(ε-caprolactone), epoxy resin, poly(ethylene oxide), poly(ethyleneglycol), and polyamide (nylon). Preferable copolymers include thefollowing: polyvinylidene-polyacrylonitrile,polyvinylidene-polyacrylonitrile-polymethylmethacrylate, andpolystyrene-polyacrylonitrile. A most preferred copolymer ispolyvinylidene-polyacrylonitrile. The term biocompatible, as used hereinin conjunction with the terms monomer or polymer, is employed in itsconventional sense, that is, to denote polymers that do notsubstantially interact with the tissues, fluids and other components ofthe body in a adverse fashion in the particular application of interest,such as the aforementioned monomers and polymers. Other suitablebiocompatible monomers and polymers will be readily apparent to thoseskilled in the art, once armed with the present disclosure.

The microspheres of the present invention are low density. By lowdensity, it is meant that the microspheres of the invention have aninternal void (cavity) volume which is at least about 75% of the totalvolume of the microsphere. Preferably, the microspheres have a voidvolume of at least about 80%, more preferably at least about 85%, evenmore preferably at least about 90%, of the total volume of themicrospheres.

The microspheres may be of varying size, provided they are low density.Suitable size microspheres include those ranging from between about 1and about 1000 microns in outside diameter, preferably between about 5and about 70 microns in outside diameter. Most preferably, themicrospheres are about 50 microns in outside diameter.

The microspheres of the invention may be prepared by various processes,as will be readily apparent to those skilled in the art, once armed withthe present disclosure, such as by interfacial polymerization, phaseseparation and coacervation, multiorifice centrifugal preparation, andsolvent evaporation. Suitable procedures which may be employed ormodified in accordance with the present disclosure to preparemicrospheres within the scope of the invention include those proceduresdisclosed in Garner et al., U.S. Pat. No. 4,179,546, Garner, U.S. Pat.No. 3,945,956, Cohrs et al., U.S. Pat. No. 4,108,806, Japan Kokai TokkyoKoho 62 286534, British Patent No. 1,044,680, Kenaga et al., U.S. Pat.No. 3,293,114, Morehouse et al., U.S. Pat. No. 3,401,475, Walters, U.S.Pat. No. 3,479,811, Walters et al., U.S. Pat. No. 3,488,714, Morehouseet al., U.S. Pat. No. 3,615,972, Baker et al., U.S. Pat. No. 4,549,892,Sands et al., U.S. Pat. No. 4,540,629, Sands et al., U.S. Pat. No.4,421,562, Sands, U.S. Pat. No. 4,420,442, Mathiowitz et al., U.S. Pat.No. 4,898,734, Lencki et al., U.S. Pat. No. 4,822,534, Herbig et al.,U.S. Pat. No. 3,732,172, Himmel et al., U.S. Pat. No. 3,594,326,Sommerville et al., U.S. Pat. No. 3,015,128, Deasy, Microencapsulationand Related Drug Processes, Vol. 20, Chs. 9 and 10, pp. 195–240(MarcelDekker, Inc., N.Y., 1984), Chang et al., Canadian J. of Physiology andPharmacology, Vol 44, pp. 115–129 (1966), and Chang, Science, Vol. 146,pp. 524–525(1964), the disclosures of each of which are incorporatedherein by reference in their entirety.

In accordance with the preferable synthesis protocol, the microspheresare prepared using a heat expansion process such as is described inGarner et al., U.S. Pat. No. 4,179,546, Garner, U.S. Pat. No. 3,945,956,Cohrs et al., U.S. Pat. No. 4,108,806, British Patent No. 1,044,680, andJapan Kokai Tokkyo Koho 62 286534. In general terms, the heat expansionprocess is carried out by preparing microspheres of an expandablepolymer or copolymer which contain in their void (cavity) a volatileliquid. The microsphere is then heated, plasticising the microsphere andvolatilizing the gas, causing the microsphere to expand to up to aboutseveral times its original size. When the heat is removed, thethermoplastic polymer retains at least some of its expanded shape.Microspheres produced by this process tend to be of particularly lowdensity, and are thus preferred. The foregoing described process is wellknown in the art, and is referred to herein as the heat expansionprocess for preparing low density microspheres.

Polymers useful in the heat expansion process will be readily apparentto those skilled in the art and include thermoplastic polymers orcopolymers, including polymers or copolymers of many of the monomersdescribed above. Preferable of the polymers and copolymers describedabove include the following copolymers:polyvinylidene-polyacrylonitrile,polyvinylidene-polyacrylonitrile-polymethylmethacrylate, andpolystyrene-polyacrylonitrile. A most preferred copolymer ispolyvinylidene-polyacrylonitrile.

Volatile liquids useful in the heat expansion process will also be wellknown to those skilled in the art and include: aliphatic hydrocarbonssuch as ethane, ethylene, propane, propene, butane, isobutane,neopentane, acetylene, hexane, heptane; chlorofluorocarbons such as

tetraalkyl silanes such as tetramethyl silane, trimethylethyl silane,trimethylisopropyl silane, and trimethyl n-propyl silane; as well asperfluorocarbons such as those having between 1 and about 9 carbon atomsand between about 4 and about 20 fluorine atoms, especially C₄F₁₀. Ingeneral, it is important that the volatile liquid not be a solvent forthe microsphere polymer or copolymer. The volatile liquid should alsohave a boiling point that is below the softening point of themicrosphere polymer or co-polymer. Boiling points of various volatileliquids and softening points of various polymers and copolymers will bereadily ascertainable to one skilled in the art, and suitablecombinations of polymers or copolymers and volatile liquids will beeasily apparent to the skilled artisan. By way of guidance, and as oneskilled in the art would recognize, generally as the length of thecarbon chain of the volatile liquid increases, the boiling point of thatliquid increases. Also, by mildly preheating the microspheres in waterin the presence of hydrogen peroxide prior to definitive heating andexpansion may pre-soften the microsphere to allow expansion to occurmore readily.

For example, to produce microspheres of the present invention,vinylidene and acrylonitrile may be copolymerized in a medium ofisobutane liquid using one or more of the foregoing modified orunmodified literature procedures, such that isobutane becomes entrappedwithin the microspheres. When such microspheres are then heated tobetween about 80° C. and about 120° C., the isobutane gas expands, whichin turn expands the microspheres. After heat is removed, the expandedpolyvinylidene and acrylo-nitrile copolymer microspheres remainsubstantially fixed in their expanded position. The resulting lowdensity microspheres are extremely stable both dry and suspended in anaqueous media. Isobutane is utilized merely as an illustrative liquid,with the understanding that other liquids which undergo liquid/gastransitions at temperatures useful for the synthesis of thesemicrospheres and formation of the very low density microspheres uponheating can be substituted for isobutane. Similarly, monomers other thanvinylidene and acrylonitrile may be employed in preparing themicrosphere.

Most preferably, the low density microspheres employed are thosecommercially available from Expancel, Nobel Industries, Sundsvall,Sweden, such as the EXPANCEL 551 DE™ microspheres. The EXPANCEL 551 DE™microspheres are composed of a copolymer of vinylidene andacrylo-nitrile which have encapsulated therein isobutane liquid. Suchmicrospheres are sold as a dry composition and are approximately 50microns in size. The EXPANCEL 551 DE™ microspheres have a specificgravity of only 0.02 to 0.05, which is between one-fiftieth andone-twentieth the density of water.

In one embodiment, the microspheres of the present invention aregas-filled. By gas-filled, it is meant that at least part of the voidvolume inside the microspheres is occupied by the gas. Preferably,substantially all of the void volume inside the microspheres is occupiedby the gas. The gas may be any type of gas, such as, for example, carbondioxide, oxygen, nitrogen, xenon, argon, neon, helium and air.Preferably, the gas is carbon dioxide, oxygen, nitrogen, xenon, argon,neon and helium. Most preferably, the gas is inert, that is, a gas thatis substantially resistant to chemical or physical action. Thegas-filled low density microspheres may be synthesized under pressuresuch that gases are solubilized in the liquid employed in microspheresynthesis. When the pressure is removed, the gas comes out of solutionto fill the microsphere void. Such microspheres can further be subjectedto a heat expansion process, as described above.

For example, to produce the gas-filled microspheres of the invention,one may copolymerize vinylidene and acrylonitrile using one or more ofthe foregoing procedures, such as phase separation/coacervationtechniques in a pressurized and/or low temperature environment (e.g., atabout 300 psi, and/or at about 0° C.) with a high concentration ofdissolved gas (e.g., dissolved nitrogen) in solution, to form a largemicrosphere containing the dissolved gas. When the pressure is removedand/or the temperature raised, the gas bubbles come out of solution,forming gas filled microspheres. Such microspheres can further besubjected to a heat expansion process, as described above.

It is preferable that the microspheres be relatively stable in thegastrointestinal tract or other body cavities during the length of timenecessary for completing an imaging examination. Low densitymicrospheres prepared from the aforementioned monomer and polymercompositions will provide such stable microspheres.

In order for these microspheres to serve as effective CT contrastagents, it is necessary for the microspheres to be mixed in solution ina substantially homogeneous suspension. This can be accomplished byusing thickening and suspending agents. A wide variety of thickening andsuspending agents may be used to a prepare the substantially homogeneoussuspensions of the microspheres. Suitable thickening and suspendingagents, for example, include any and all biocompatible agents known inthe art to act as thickening and suspending agents. Particularly usefulare the natural thickening and suspending agents alginates, xanthan gum,guar, pectin, tragacanth, bassorin, karaya, gum arabic, casein, gelatin,cellulose, sodium carboxymethylcellulose, methylcellulose,methylhydroxycellulose, bentonite, colloidal silicic acid, andcarrageenin, and the synthetic thickening and suspending agentspolyethylene glycol, polypropylene glycol, and polyvinylpyrrolidone. Asthose skilled in the art would recognize, once armed with the presentdisclosure, the suspending agents may be formulated, if desired, to beeither less dense than water or of neutral density, so as to notsubtract from the density lowering capabilities of the microspheres. Forexample, a cellulose suspension may have a somewhat lower density thanwater, e.g., a 2 weight % cellulose solution with 0.25 weight % xanthangum has a density of 0.95. The thickening and suspending agents may beemployed in varying amounts, as those skilled in the art wouldrecognize, but preferably are employed in amounts of about 0.25 to about10 weight % preferably about 0.5 to about 5 weight % of the contrastmedium.

The substantially homogeneous, aqueous suspension of low densitymicrospheres of the invention are useful as CT contrast agents. Theseagents are capable of producing negative contrast in thegastrointestinal tract or in other body cavities, providing effectivecontrast enhancement and improved visualization in these areas of thebody. Specifically, the present invention is directed to a method ofproviding an image of or detecting diseased tissue in thegastrointestinal region and other body cavities of a patient, the methodcomprising administering to the patient a contrast medium comprising asubstantially homogeneous aqueous solution of low density microspheres,and scanning the patient using computed tomography imaging to obtainvisible images of the gastrointestinal region or other body cavities orof diseased tissue in these areas of the body. The phrasegastrointestinal region or gastrointestinal tract, as used herein,includes the region of a patient defined by the esophagus, stomach,small and large intestines, and rectum. The phrase other body cavities,as used herein, includes any region of the patient, other than thegastrointestinal region, having an open passage, either directly orindirectly, to the external environment, such regions including thesinus tracts, the fallopian tubes, the bladder, etc. The patient can beany type of mammal, but most preferably is a human. As one skilled inthe art would recognize, administration of the contrast medium to thepatient may be carried out in various fashions, such as orally,rectally, or by injection. When the region to be scanned is thegastrointestinal region, administration of the contrast medium of theinvention is preferably carried out orally or rectally. When other bodycavities such as the fallopian tubes or sinus tracts are to be scanned,administration is preferably by injection. As would also be recognizedby one skilled in the art, wide variations in the amounts of the gasfilled microspheres can be employed in the methods and kits of theinvention, with the precise amounts varying depending upon such factorsas the mode of administration (e.g., oral, rectal, by injection), andthe specific body cavity and portion thereof for which an image issought (e.g., the stomach of the gastrointestinal tract). Typically,dosage is initiated at lower levels and increased until the desiredcontrast enhancement is achieved.

For CT imaging, it is generally desirable to decrease the density of thelumen of the gastrointestinal tract or other body cavities to at leastabout −30 HU, the maximum decrease being limited by the practical amountof the microspheres which may be suspended in the aqueous media andingested by the patient. In general, a decrease in HU to between about−30 HU and about −150 HU is sufficient to mark the inside of the bowelor other body cavity. By way of general guidance, and as a rough rule ofthumb, to decrease the density of the microsphere aqueous suspension toabout −150 HU, the microspheres must occupy about 15% of the totalvolume of the aqueous suspension. To achieve a density of about −50 HU,the microspheres must occupy about 5% of the total volume of thesolution. The volume of contrast agent administered to the patient istypically between about 50 to about 1000 cc. Using the EXPANCEL 551 DE™microspheres as a model, it has been found that about 0.6 grams of thedry 50 micron spheres in 100 cc of aqueous suspension is sufficient todecrease the density of the suspension to nearly −150 HU.

It should be noted that smaller microspheres are generally more stablein suspension, but usually have higher specific gravity than largermicrospheres. Therefore, for CT, the size and particular microspheres,as well as the suspending media (thickening and suspending agents)should selected to minimize specific gravity, while maximizing thestability of the suspension.

The contrast medium utilized of the present invention may also beemployed with other conventional additives suitable for use in theapplications contemplated for the subject invention.

Where gastrointestinal applications are concerned, such additivesinclude conventional biocompatible anti-gas agents, osmolality raisingagents, gastrointestinal transit agents (the later agents serving todecrease the gastrointestinal transit time and increase the rate ofgastrointestinal emptying) and, in some instances, gas-forming agents.

As used herein the term anti-gas agent is a compound that serves tominimize or decrease gas formation, dispersion and/or adsorption. Anumber of such agents are available, including antacids, antiflatulents,antifoaming agents, and surfactants. Such antacids and antiflatulentsinclude, for example, activated charcoal, aluminum carbonate, aluminumhydroxide, aluminum phosphate, calcium carbonate, dihydroxyaluminumsodium carbonate, magaldrate magnesium oxide, magnesium trisilicate,simethicone, sodium carbonate, loperamide hydrochloride, diphenoxylate,hydrochloride with atropine sulfate, Kaopectate™ (kaolin) and bismuthsalts. Suitable antifoaming agents useful as anti-gas agents includesimethicone, protected simethicone, siloxyalkylene polymers, siloxaneglycol polymers, polyoxypropylene-polyoxyethylene copolymers,polyoxyalkylene amines and imines, branched polyamines, mixedoxyalkylated alcohols, finely divided silica either alone or mixed withdimethyl polysiloxane, sucroglycamides (celynols), polyoxylalkylatednatural oils, halogenated silicon-containing cyclic acetals, laurylsulfates, 2-lactylic acid esters of unicarboxylic acids, triglycerideoils. Particles of polyvinyl chloride or silica may also function asanti-foaming agents in the subject invention. Suitable surfactantsinclude perfluorocarbon surfactants, such as, for example, DuPont Zonyl™perfluoroalkyl surfactants known as Zonyl™ RP or Zonyl™ NF, availablefrom DuPont, Chemicals and Pigments Division, Jackson Laboratory,Deepwater, N.J. 08023. Of course, as those skilled in the art willrecognize, any anti-gas agents employed must be suitable for use withinthe particular biological system of the patient in which it is to beused. The concentration of such anti-gas agents may vary widely, asdesired, as will be readily apparent to those skilled in the art.Typically, however, such agents are employed in concentrations ofbetween about 20 and about 2000 ppm, most preferably in concentrationsbetween about 50 and about 1000 ppm.

Suitable osmolality raising agents include polyols and sugars, forexample, mannitol, sorbitol, arabitol, xylitol, glucose, sucrose,fructose, dextrose, and saccharine, with mannitol and sorbitol beingmost preferred. The concentration of such osmolality raising agents mayvary, as desired, however, generally a range of about 5 to about 70 g/l,preferably about 30 to about 50 g/l of the contrast medium. Suchcompounds may also serve as sweeteners for the ultimate formulation, ifdesired.

Gastrointestinal transit agents include algin, as well as many of thecompounds listed above as thickening and suspending agents, with alginbeing most preferred. The amount of such agents will, of course, vary asthose skilled in the art will recognize, but generally will be employedin an amount of between about 5 and about 40 mmol/l.

In some applications, it may be helpful to incorporate gas-formingagents into the contrast medium. Gas-forming agents include sodiumbicarbonate, calcium carbonate, aminomalonate, and the like, which willform gas, for example, upon introduction into the gastro-intestinaltract. Such gas-forming agents will serve to distend thegastrointestinal tract and create a form of “double contrast” betweenthe gas and the low density microspheres.

Kits useful for computed tomography imaging of the gastrointestinalregion or other body cavities in accordance with the present inventioncomprise low density microspheres, and a thickening or suspending agent,in addition to conventional computed tomography imaging kit components.Such conventional computed tomography kit components will be readilyapparent to those skilled in the art, once armed with the presentdisclosure.

Where imaging of the gastrointestinal region is contemplated, suchcomputed tomography kit components may include, for example, anti-gasagents, osmolality raising agents, gastrointestinal transit agents and,in some instances, gas-forming agents.

The computed tomography imaging principles and techniques which areemployed are conventional and are described, for example, in ComputedBody Tomography, Lee, J. K. T., Sagel, S. S., and Stanley, R. J., eds.,Ch. 1, pp. 1–7 (Raven Press, N.Y. 1933). Any of the various types ofcomputed tomography imaging devices can be used in the practice of theinvention, the particular type or model of the device not being criticalto the method of the invention.

The present invention is further described in the following Examples.Examples 1–7 are prophetic examples based at least in part on theteachings of Garner, U.S. Pat. No. 3,945,956, and describe thepreparation of microspheres by a heat expansion process. Examples 8–9are actual examples that describe the preparation of contrast media ofthe invention. The following Examples are not to be construed aslimiting the scope of the appended Claims.

EXAMPLES Example 1

A vessel is filled with 50 parts by weight of deionized water and 6parts by weight of a 25 percent by weight aqueous colloidal silicadispersion. A mixture of 0.3 parts by weight of a 10 weight percentsolution of diethylamine-adipic acid copolymer is added to the above. Acondensation reaction occurs creating a mixture having a viscosity ofabout 95 centipoise at a temperature of about 27° C. Potassiumdichromate (0.05 parts by weight) is added to the aqueous phase as awater phase polymerization inhibitor. Sodium chloride (1 part by weight)is also present in the water phase; hydrochloric acid is used to adjustthe pH of the aqueous phase to 4.0. Styrene (15 parts by weight),acrylonitrile (10 parts by weight), a mixture of diethylbenzene anddivinylbenzene (0.21 parts by weight comprising a 55:45 percent mixtureof each respectively), 6.25 parts by weight of isobutane and 0.07 partsby weight of secondary butyl peroxydicarbonate. The oil phase is addedto the water phase with violent agitation created by a shearing bladerotating at 10,000 RPM employing a mixing blender. After the materialhas reacted for about 30 minutes, the mixture is poured into a citratebottle and capped. The material is maintained at about 50° C. in thecitrate bath for about 24 hours and agitated throughout this time. Atthe end of 24 hours, the reaction bottle is cooled and the material isremoved, washed and dried. A portion of the microspheres are set asideand the remainder are heated in an air oven for a period of about 30minutes at about 150° C. A sample of the dry unexpanded and dry expandedmicrospheres are then studied by a Coulter Counter. The dry unexpandedmicrospheres have a size of about 2 to 12 microns. About half of themicrospheres exposed to the heating process show expansion.

Example 2

The procedures of Example 1 are substantially repeated with theexception that 1 part by weight of methanol is added to the reactionmixture. The dry unexpanded and dry heat expanded microspheres are thenstudied by Coulter Counter. The dry unexpanded microspheres measureabout 8 to 10 microns in size. Essentially all the microspheres exposedto heat expand.

Example 3

The procedures of Example 2 are substantially repeated except that aftersynthesis of the microspheres, a slurry of the microspheres is added toan aqueous solution containing 35 weight percent hydrogen peroxide. Thisslurry is heated to a temperature of about 50° C. for about 3.5 hoursand subsequently cooled and air-dried. A portion of the microspheres isthen added to water and heated to a temperature of about 75° C. withvigorous stirring for about 30 seconds. Study with Coulter Counter showsthat pretreatment with hydrogen peroxide enables a lower temperature andbriefer period of heating to be used for definitive heating andexpansion.

Example 4

The procedures of Example 1 are substantially repeated with theexception that 5 parts by weight of ethanol are included in the reactionmixture forming the microspheres. Coulter Counter shows that the dryunexpanded particles have diameters of about 24 to 28 microns. Whenheated, essentially all of the microspheres expand.

Example 5

The procedures of Example 1 are substantially repeated with theexception that in place of methanol, 1 part by weight of normal butanolis used. The diameter of the dry unexpanded microspheres is about 10 to12 microns and on heating, essentially all of the microspheres expand.

Example 6

The procedures of Example 1 are substantially repeated with theexception that the volatile liquid isobutane is replaced withperfluorocarbon liquid (C₄F₁₀). The remainder of the process is similar.The resulting microspheres are filled with perfluorocarbon liquid ratherthan isobutane.

Example 7

The procedures of Example 1 are substantially repeated with theexception that the reaction is conducted in a pressurized vesselenabling pressurization with gas and simultaneous agitation (agitationaccomplished with either sonication or shearing blades within thedevice). As the microspheres are formed within the device, the vessel ispressurized to about 300 psi with nitrogen gas. The vessel is thendepressurized, allowing the gas to come out of solution. Themicrospheres are then subjected to heat as substantially described inExample 1.

Example 8

A suspension of 2% of 22 micron fiber length cellulose in 0.25% xanthangum in water was prepared. Scans by CT showed a CT density of about −45HU for the cellulose suspension. EXPANCEL 551 DE™polyvinylidene-polyacrylonitrile microspheres, 50 microns in size, werethen suspended in the aqueous cellulose suspension at a concentration of0.4 grams of microspheres per 100 ml of cellulose suspension usingvigorous shaking. The resulting suspension remained substantiallyhomogeneous for about 10 minutes. The suspension was again shakenvigorously to render it substantially homogeneous and scannedimmediately by CT. The resulting CT density as measured by the scannerwas about −96 HU.

Example 9

A suspension of 1% algin was prepared. EXPANCEL 551 DE™ microsphereswere added to the algin suspension in an amount of about 0.2 grams ofmicrospheres per deciliter of algin suspension, using vigorous shaking,to form a substantially homogeneous suspension. The resulting suspensionwas found to have much greater stability than the cellulose/microspheresuspension of Example 1. The algin/microsphere suspension was thenscanned by CT, with the density as measured by the scanner being about−40 HU.

Various modifications of the invention in addition to those shown anddescribed herein will be apparent to those skilled in the art from theforegoing description. Such modifications are also intended to fallwithin the scope of the appended claims.

1. A composition comprising low density microspheres having an outsidediameter of from about 5 to about 70 microns, said microspherescomprising a polymer and having a void containing the vapor of avolatile liquid perfluorocarbon, wherein said microspheres, whencombined with an aqueous solution to form a contrast agent, are suitablefor administration to a patient either orally, rectally or by injection.2. A composition according to claim 1 wherein said polymer comprises asynthetic polymer or copolymer prepared from the group of monomersconsisting of acrylic acid, methacrylic acid, ethyleneimine, crotonicacid, acrylamide, ethyl acrylate, methyl methacrylate, 2-hydroxyethylmethacrylate, lactic acid, glycolic acid, ε-caprolactone, acrolein,cyanoacrylate, bisphenol A, epichlorhydrin, hydroxyalkylacrylates,siloxane, dimethylsiloxane, ethylene oxide, ethylene glycol,hydroxyalkyl-methacrylates, N-substituted acrylamides, N-substitutedmethacrylamides, N-vinyl-2-pyrrolidone, 2,4-pentadiene-1-ol, vinylacetate, acrylonitrile, styrene, p-amino-styrene,p-amino-benzyl-styrene, sodium styrene sulfonate, sodium 2-sulfoxyethylmethacrylate, vinyl pyridine, aminoethyl methacrylates,2-methacryloyloxy-trimethylammonium chloride,N,N′-methylenebisacrylamide, ethylene glycol dimethacrylates,2,2′-(p-phenylenedioxy)-diethyl dimethacrylate, divinylbenzene,triallylamine, and methylenebis-(4-phenyl-isocyanate).
 3. A compositionaccording to claim 2 wherein said microspheres comprise syntheticpolymers or copolymers prepared from the group of monomers consisting ofacrylic acid, methacrylic acid, ethyleneimine, crotonic acid,acrylamide, ethyl acrylate, methyl methacrylate, 2-hydroxyethylmethacrylate, lactic acid, glycolic acid, ε-caprolactone, acrolein,cyanoacrylate, bisphenol A, epichlorhydrin, hydroxyalkylacrylates,siloxane, dimethylsiloxane, ethylene oxide, ethylene glycol,hydroxyalkyl-methacrylates, N-substituted acrylamides, N-substitutedmethacrylamides, N-vinyl-2-pyrrolidone, 2,4-pentadiene-1-ol, vinylacetate, acrylonitrile, styrene, p-amino-styrene,p-amino-benzyl-styrene, sodium styrene sulfonate, sodium2-sulfoxyethylmethacrylate, vinyl pyridine, aminoethyl methacrylates,and 2-methacryloyloxy-trimethylammonium chloride.
 4. A compositionaccording to claim 1 wherein said microspheres comprise syntheticpolymers or copolymers selected from the group consisting of polyacrylicacid, polyethyleneimine, polymethacrylic acid, polymethylmethacrylate,polysiloxane, polydimethylsiloxane, polylactic acid,poly(ε-capro-lactone), epoxy resin, poly(ethylene oxide), poly(ethyleneglycol), polyamide, polyvinylidene-polyacrylonitrile,polyvinylidene-poly-acrylonitrile-polymethylmethacrylate, andpolystyrene-polyacrylonitrile.
 5. A composition according to claim 1wherein said microspheres are prepared by a heat expansion process.
 6. Acomposition according to claim 1 where said perfluorocarbon has between1 and about 9 carbon atoms and between 4 and about 20 fluorine atoms. 7.A composition according to claim 6 wherein said perfluorocarbon hasbetween 1 and 4 carbon atoms and between 4 and 10 fluorine atoms.
 8. Acomposition according to claim 6 wherein said perfluorocarbon hasbetween 4 and about 9 carbon atoms and between 10 and about 20 fluorineatoms.
 9. A composition according to claim 6 wherein saidperfluorocarbon is C₄F₁₀.